The congressman leading the Republican investigation into last year’s terrorist attacks on U.S. facilities in Benghazi, Libya, on Friday ordered retired Ambassador Thomas Pickering to submit to questioning behind closed doors next week over an internal State Department review Pickering helped lead into the attacks.

The congressman leading the Republican investigation into last year’s terrorist attacks on U.S. facilities in Benghazi, Libya, on Friday ordered retired Ambassador Thomas Pickering to submit to questioning behind closed doors next week over an internal State Department review Pickering helped lead into the attacks.

The subpoena from Rep. Darrell Issa, R-Calif., the chairman of the House Oversight and Government Reform Committee, represents the latest chapter in an acrimonious battle between the Obama administration and Republicans over the attacks, with the Republicans claiming the White House tried to cover up the role of Islamist extremists to protect President Barack Obama’s re-election bid.

Pickering had told Issa in a letter that he would be happy to appear at a public session of Issa’s committee but would not submit to questioning in private beforehand. Issa instead ordered Pickering to appear for a private deposition May 23 but said that he’d consider withdrawing the subpoena if the veteran diplomat agreed to voluntarily appear.

Four Americans, including the U.S. ambassador to Libya, Christopher Stevens, died in the attacks, which struck a U.S. diplomatic compound and, hours later, a CIA annex nearby. The attacks occurred on the 11th anniversary of the Sept. 11, 2001, terrorist attacks in the United States.

Pickering and retired Adm. Mike Mullen, a former chairman of the Joint Chiefs of Staff, led a review that in December cited the State Department’s leadership for failing to ensure adequate protection for U.S. personnel in Libya even as violence by al Qaida-linked militants and other Islamists grew following the U.S.-backed ouster of the late dictator Moammar Gadhafi.

Issa issued his subpoena a day after Pickering and Mullen said they were ready to testify in public but rejected as an “inappropriate precondition” a request that they first submit to a closed-door deposition on the work of their Accountability Review Board.

In a letter addressed only to Pickering, Issa on Friday accused the pair of “attempting to limit” his committee’s “understanding of the Accountability Review Board by refusing to participate in a voluntary transcribed interview prior to testifying publicly.”

“In light of your continuing refusal to appear voluntarily for a transcribed interview, however, I have found it necessary to issue a subpoena to compel your appearance at a deposition,” Issa said.

Issa, who clashed with Pickering on a television talk show Sunday, didn’t explain why he didn’t subpoena Mullen.

Pickering, asked about the subpoena at a conference on the Middle East at which he was speaking, said that he had just heard about it and first wanted to “talk to my friends at the State Department and seek legal advice.”

Asked if he’d expected to find himself embroiled in a political controversy when he agreed to co-chair the review board, Pickering replied with a slight chuckle, “I had no idea but a deep sense of – put it this way – potential concern that anything that was so politicized at the beginning would not necessarily end up with some political ramifications in the end.”

Rep. Elijah Cummings of Maryland, the senior Democrat on Issa’s committee, denounced the subpoena as “a stark example of extreme Republican overreach and the shameful politicization of this tragedy.”

In his letter to Pickering, Issa said that he wanted him and Mullen to first answer questions in a closed-door deposition so that the committee could conduct a “fully informed” hearing on the review board’s work, which Issa and other Republicans have harshly criticized.

“The ARB worked behind closed doors. It did not record its interviews. No transcripts of ARB interviews exist. Even now, months after the ARB report was released, the ARB’s investigative process remains opaque,” Issa said.

He also pointed out that three State Department officials who testified before his committee last week also raised concerns about the board’s work, including a failure to hold senior officials accountable.